1. Technical Field
The invention relates to color printing. More particularly, the invention relates to color printing with regard to color separation, color variation, color characterization, and gamut mapping.
2. Description of the Background Art
Modern color printing relies upon various well known techniques, such as color separation and half tone techniques.
Color Separation
The process of color separation starts by separating the original artwork into red, green, and blue components, for example by a digital scanner. Before digital imaging was developed, the traditional method of doing this was to photograph the image three times, using a filter for each color. However this is achieved, the desired result is three grey scale images, which represent the red, green, and blue (RGB) components of the original image.
The next step is to invert each of these separations. When a negative image of the red component is produced, the resulting image represents the cyan component of the image. Likewise, negatives are produced of the green and blue components to produce magenta and yellow separations, respectively. This is done because cyan, magenta, and yellow are subtractive primaries which each represent two of the three additive primaries (RGB) after one additive primary has been subtracted from white light.
Cyan, magenta, and yellow are the three basic colors used for color reproduction. When these three colors are variously used in printing, the result should be a reasonable reproduction of the original, but in practice this is not the case. Due to limitations in the inks, the darker colors are dirty and muddied. To resolve this, a black separation is also created, which improves the shadow and contrast of the image. Numerous techniques exist to derive this black separation from the original image; these include grey component replacement, under color removal, and under color addition. This printing technique is referred to as CMYK (the “K” stands for Key, a traditional word for the black printing plate).
Today's digital printing methods do not have the restriction of a single color space that traditional CMYK processes do. Many presses can print from files that were ripped with images using either RGB or CMYK modes. The color reproduction abilities of a particular color space can vary; the process of obtaining accurate colors within a color model is called color matching.
Color Characterization
To describe the behavior of various output devices, they must be compared (measured) in relation to a standard color space. Often a step called linearization is performed first to undo the effect of gamma correction that was done to get the most out of limited 8-bit color paths. Instruments used for measuring device colors include colorimeters and spectrophotometers. As an intermediate result, the device gamut is described in the form of scattered measurement data. The transformation of the scattered measurement data into a more regular form, usable by the application, is called profiling. Profiling is a complex process involving mathematics, intense computation, judgment, testing, and iteration. After the profiling is finished, an idealized color description of the device is created. This description is called a profile.
Gamut Mapping
Because different devices do not have the same gamut, they need some rearrangement near the borders of the gamut. Some colors must be shifted to the inside of the gamut, as they otherwise cannot be represented on the output device and would simply be clipped. For instance the dark highly saturated purplish-blue color of a typical computer monitor's “blue” primary is impossible to print on paper with a typical CMYK printer. The nearest approximation within the printer's gamut is much less saturated. Conversely, an inkjet printer's “cyan” primary, a saturated mid-brightness greenish-blue, is outside the gamut of a typical computer monitor. A color management system can use various methods to achieve desired results and give experienced users control of the gamut mapping behavior.
These and other techniques, provide a technical pallet with which color images may be printed. Unfortunately, these techniques alone often create as many problems as they solve. It would be advantageous to provide refined use of such techniques for color printing with regard to such aspects of color printing as color separation, color variation, color characterization, and gamut mapping.